1. Field of the Invention
The present invention relates to devices for measuring the concentration of toner in a liquid developer by detecting the amount of light passing through the liquid developer, to image forming apparatuses provided with such devices, and to toner concentration measuring methods.
2. Description of Related Art
Developing methods used in image forming apparatuses are broadly classified into dry developing and wet developing. Previously, dry developing was widely used from the viewpoint of ease of handling and reduction in size and weight. In recent years, however, wet developing has been receiving new attention from the viewpoint of, for example, improving the quality of images.
In wet developing, a liquid developer made of a carrier liquid, such as silicone or paraffin, and toner particles suspended in the carrier liquid is used to develop an electrostatic latent image formed on the surface of a photoconductor based on image data. To obtain an image of high quality by wet developing, it is important to keep the toner concentration in the liquid developer at an appropriate level.
Therefore, in JP-A-2005-315948, a recess is formed in the inner wall surface of a liquid developer reservoir, and a phototransmitter is provided in the bottom of the recess. In addition, a movable body is provided in such a way that it can move close to or away from the inner wall surface, and a photoreceptor is provided in the movable body in a face thereof facing the phototransmitter. When toner concentration is measured, the movable body is moved so as to make close contact with the inner wall surface, such that a liquid developer is trapped in the narrow space between the movable body and the inner wall surface. The liquid developer trapped therein is irradiated with light emitted from the phototransmitter, and the amount of light passing through the liquid developer and reaching the photoreceptor is detected. In this way, the toner concentration is measured.
In this case, when the toner concentration of a liquid developer using toner having a low light transmission, such as black, is measured, a thick layer of liquid developer prevents measurement light from passing therethrough. This makes high-accuracy measurement impossible. To avoid this problem, it is necessary to trap the liquid developer as thin as on the order of several tens of μm. However, due to the difficulty in forming a recess having a depth of several tens of μm, a spacer is used instead for forming a narrow space in which a liquid developer is trapped. FIGS. 12A and 12B each show a toner concentration measuring device in which a narrow space is formed by using spacers. The toner concentration measuring device shown in FIGS. 12A and 12B has a light-emitting portion 101 and a light-receiving portion 102. The light-receiving portion 102 is provided with a light incidence surface 122a and a light-receiving member 105 that is so disposed as to face the light-emitting portion 101 with the light incidence surface 122a between them. The light-receiving portion 102 is attached to the measuring tank 103 in such a way that the light incidence surface 122a is flush with the bottom of the measuring tank 103. On the other hand, the light-emitting portion 101 has a light-emitting member 104 and a light emergence surface 112a from which light emitted from the light-emitting member 104 emerges, and the light-emitting portion 101 is attached in such a way as to permit the light emergence surface 112a to move up and down with respect to the light incidence surface 122a. At the bottom of the measuring tank 103 near the light incidence surface 122a are provided spacers 107a and 107b, each being formed of a metal plate that is several tens of μm thick, in such a way that they face each other with the light incidence surface 122a disposed between them.
As shown in FIG. 12B, when toner concentration is measured, the light-emitting portion 101 is moved downward until the bottom surface of the light-emitting portion 101 makes contact with the top faces of the spacers 107a and 107b. This creates a space having a predetermined thickness (equal to the thickness of the spacers 107a and 107b) between the light emergence surface 112a and the light incidence surface 122a. The liquid developer (not shown) trapped in this space is irradiated with light emitted from the light-emitting portion 101, and the amount of light passing through the liquid developer and reaching the light-receiving member 105 through the light incidence surface 122a is detected by the light-receiving member 105. In this way, the toner concentration is measured.
Disadvantageously, by the former method in which the space is formed by the formation of a recess, the toner may accumulate in the recess formed in the inner wall surface, making it impossible to measure the toner concentration with high accuracy. By the latter method in which the space is formed by using spacers, as shown in FIG. 13, toner T may be stuck between the bottom of the measuring tank 103 and the spacers 107a and 107b, making the space wider than a predetermined value. To avoid this problem, the spacers 107a and 107b may be bonded to the bottom of the measuring tank 103 with an adhesive or the like. However, it is technically difficult to control the thickness of a coating of adhesive in micrometers.